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Re: [Getfem-users] Example of a simple mixed-problem, or problem with t

From: Renard Yves
Subject: Re: [Getfem-users] Example of a simple mixed-problem, or problem with two variables
Date: Sun, 05 Jul 2009 22:39:22 +0200
User-agent: Dynamic Internet Messaging Program (DIMP) H3 (1.1)

Dear Jehanzeb,

You have to use a volumic assembly . An example for the matrix of the Laplace operator is

gf_asm('volumic',['a=data(#2); ',...
'M(#1,#1)+=sym(comp(Grad(#1).Grad(#1).Base(#2))(:,i,:,i,j).a(j))'], mim, mf, mf_data, A);

A matrix for a mixed term can be obtained in a same way with two mf :

gf_asm('volumic', 'M(#1,#1)+=comp(Grad(#1).Base(#2))(:,i,i,:)', mim, mf1, mf2);

where mf1 should be a vectorial fem (for more details see the Getfem++ documentation).


Jehanzeb Hameed <address@hidden> a écrit :

I had a look at the Stokes example. It uses model bricks to form the
system. From the documentation, it seems using model bricks implicitly
chooses the bilinear variational form. I would like to define my own
bilinear form to form the matrix, in which I can define terms like
div(q) + c u  (q = vector, c = constant, u = another variable besides
q). How would I go about this?


On Sun, Jul 5, 2009 at 11:05 AM, Iago Barbeiro<address@hidden> wrote:
Dear Jehanzeb,

I think you may start by looking the Stokes example.
It deals with two variables (û=uî+v^j and p) and has the term div(û).dp.
Bon courage!


On Sun, Jul 5, 2009 at 6:07 AM, Jehanzeb Hameed <address@hidden>


Is there a simple example in getfem where assembly is for two
variables? E.g. we may want to solve for "u" and "q" (with an equation
defining the relationship between u and q) . Such a case arises in
mixed methods.  Is there an example for mixed-poisson problem
somewhere? I know mixed-elasticity problem is given with getfem, but I
am not familiar with that particular problem.

In particular, I am not sure how to refer to "q" and "u" in
generic_assembly routines. Say my weak form involves div(q) . v ? How
will I write this in "assem.set" routine? (I am guessing thats what I
am supposed to do).


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